Multicasting technology is a relative efficient way to transmit multimedia video and audio data in internet or intranet among a designated group of subscribers. The examples to which the multicasting technology is applied include live video conference, web radio, web television, etc. Due to the extremely large quantity of data transmission in multimedia video and audio files, the bandwidth of the entire network is likely to be occupied by the multimedia video and audio data so as to paralyze the network when video and audio casting behavior proceeds in the network. Under this circumstance, an IP multicasting way is preferably performed to accomplish such video and audio casting behavior. The basic rules of the multicasting behavior are generally stipulated in the Internet Group Management Protocol (IGMP) described in the Request for Comment 1112 (RFC 1112). Switch hubs with IGMP functions can directly transmit IP multicasting data to the destination ports while allowing the other ports to share the bandwidth of the network normally. Common switch hubs, however, cannot distinguish the IP multicasting data.
Please refer to FIG. 1 which is a schematic diagram showing a video-on-demand (VOD) network structure capable of supporting IGMP multicasting functions. The backbone router 10 is connected to one or more workstations such as a live information provider 101 and/or a video-on-demand (VOD) server 102. The information can be transferred to the designated group of subscribers who may reside in different countries in the world via the IGMP multicasting technology provided the switch hubs with IGMP functions are used. It is assumed that the designated group of subscribers sharing the VOD server 102 include subscribers 111 and 112 connected to the first switch hub 11, a subscriber 121 connected to the second switch hub 12, and a subscriber 133 connected to the third switch hub 13, with a multicast group ID 224. 1. 1. 5. When the switch hubs 11, 12 and 13 receive an external packet, a so-called snooping operation is performed to correlate the destination IP address of the received packet with the IP multicast table built in each of the switch hubs in order to find out a port mask for the subsequent data transmission. For example, when the first switch hub 11 having eight ports receives a packet from the VOD server 102, a look-up-table operation is performed according to the destination IP address, i.e. 224. 1. 1. 5. From the information in the table, it is generated that only the subscribers 111 and 112 are the subscribers of the VOD server 102. Therefore, the resulting port mask is 11000000. The packet from the VOD server 102 can be outputted to the first and the second ports corresponding to the subscribers 111 and 112 only, thereby accomplishing the multicasting operation.
A virtual local area network (VLAN), on the other hand, is a network technology developed for the network security issue. In the VLAN technology, some subscribers are grouped to form an isolated broadcast domain. The broadcasting or group casting information packets from any of the workstations in the VLAN will be sent to the VLAN members only. By using network management software, it is possible to divide the subscribers of the same physical broadcast domain into several logical broadcast domains, or logically group the subscribers of different physical broadcast domains into the same broadcast domain. The VLAN technology allows the subscribers under the same Ethernet structure to keep privacy and network security, and the subscribers residing at different locations to be linked together. The virtual feature results in good flexibility.
The VLAN units can be ports or multimedia access controller (MAC) addresses. For the MAC-based VLAN, the transmission path of the data packets is determined according to a VLAN table correlating VLAN ID or VID stored in the switch hub. Please refer to FIG. 2 that schematically shows a partial format of a data packet wherein a VLAN tag is specified. The 4-byteVLAN tag between the source MAC (SMAC) address and the Ethernet type includes a VLAN ID or VID of 12 bits.
In order to support the VLAN functions, many newly developed network cards recognize VLAN tags, and are called VLAN cards hereinafter. The VLAN cards, however, do not accept the data packet without the VLAN tag. In other words, for the VOD network structure supporting IGMP multicasting functions as shown in FIG. 1, the multicasted data packets from the IGMP switch hubs will be discarded by the VLAN cards when some subscribers use VLAN cards. Accordingly, the data transmission cannot be accomplished.